The present invention relates generally to motors/generators and, more particularly, to high speed switched reluctance machines capable of starting a prime mover as well as generating electrical power for use on aircraft.
The aerospace industry has consistently driven the leading edge of technology with the requirement for lightweight, high efficiency, high reliability equipment. The equipment must be lightweight because each additional pound of weight translates directly into increased fuel burn, and therefore, a higher cost of ownership and shorter range. The need for high efficiency results from the fact that each additional cubic inch required for equipment displaces the amount of revenue-generating cargo and passengers that can be carried on an aircraft. High reliability is important because every minute of delay at the gate increases the cost of ownership, and likewise, increases passenger frustration.
For aircraft electric power generation systems, these pressures have precipitated great advancements in technology, but have also caused problems. Aircraft have typically used synchronous brushless AC generators or permanent magnet generators for electric power generation needs. Unfortunately, both of these types of generators require components which can fail due to the conditions under which they are required to operate (usually mounted directly on the aircraft jet engine).
As an alternative to the use of the synchronous AC or the permanent magnet generator, a switched reluctance machine can be used. A switched reluctance machine is an inherently low cost machine, having a simple construction which is capable of very high speed operation, thus yielding a more lightweight design. The rotor of the switched reluctance machine is constructed from a simple stack of laminations making it very rugged and low cost without the containment problems associated with rotor windings or permanent magnets. Further, the rotor does not require rotating rectifiers, which contribute to failures, as in the AC synchronous machine.
In order to properly operate a switched reluctance machine, it has been found necessary in the past to determine the rotor position in order to properly commutate the currents flowing in the phase windings of the machine. Resolvers are used, particularly in high speed systems, or sometimes encoders in lower speed systems, to obtain a measure of rotor position. However, resolvers and required associated apparatus (chiefly, a resolver-to-digital converter and an excitation circuit) are expensive and both resolvers and encoders are sources of single point failure.
In order to obviate the need for position sensors, such as resolvers or encoders, sensorless operational techniques have been developed. The most trivial solution to sensorless operation is to control the switched reluctance machine as a stepper motor in the fashion disclosed in Bass, et al. U.S. Pat. No. 4,611,157 and MacMinn U.S. Pat. No. 4,642,543. In an alternative technique, machine inductance or reluctance is detected and utilized to estimate rotor position. Specifically, because the phase inductance of a switched reluctance machine varies as a function of angle from alignment of the stator pole for that phase and a rotor pole, a measurement of instantaneous phase inductance can be utilized to derive an estimate of rotor position. See MacMinn, et al. U.S. Pat. No. 4,772,839, MacMinn, et al. U.S. Pat. No. 4,959,596, Harris xe2x80x9cPractical Indirect Position Sensing for a Variable Reluctance Motor,xe2x80x9d Masters of Science Thesis, MIT, May 1987, Harris, et al. xe2x80x9cA Simple Motion Estimator for Variable Reluctance Motors,xe2x80x9d IEEE Transactions on Industrial Applications, Vol. 26, No. 2, March/April, 1990, and MacMinn, et al. xe2x80x9cApplication of Sensor Integration Techniques to Switched Reluctance Motor Drives,xe2x80x9d IEEE Transactions on Industry Applications, Vol. 28, No. 6, November/December, 1992.
In a further technique, phase inductance can be determined using a frequency modulation approach whereby a non-torque producing phase forms part of a frequency modulation encoder. See Ehsani, et al. xe2x80x9cLow Cost Sensorless Switched Reluctance Motor Drives for Automotive Applications,xe2x80x9d Texas AandM Power Electronics Laboratory Report (date unknown), Ehsani, et al. xe2x80x9cAn Analysis of the Error in Indirect Rotor Position Sensing of Switched Reluctance Motors,xe2x80x9d IEEE Proceedings IECON ""91, Ehsani xe2x80x9cA Comparative Analysis of SRM Discrete Shaft Position Sensor Elimination by FM Encoder and Pulsed Impedance Sensing Schemes,xe2x80x9d Texas AandM Power Electronics Laboratory Report, (date unknown) and Ehsani, et al. xe2x80x9cNew Modulation Encoding Techniques for Indirect Rotor Position Sensing in Switched Reluctance Motors,xe2x80x9d IEEE Transactions on Industry Applications, Vol. 30, No. 1, January/February, 1994.
A model-based approach to rotor position estimation has been developed by General Electric Company and is disclosed in Lyons, et al. xe2x80x9cFlux/Current Methods for SRM Rotor Position Estimation,xe2x80x9d Proceedings of IEEE Industry Applications Society Annual Meeting, Vol. 1, 1991, and Lyons, et al. U.S. Pat. No. 5,097,190. In this technique, a multi-phase lumped parameter model of the switched reluctance machine is developed and utilized. However, the model has been developed only for a three-phase machine wound in a north-south-north-south-north-south configuration.
A position estimation subsystem has been developed by the assignee of the instant application and includes a relative angle estimation circuit, an angle combination circuit and an estimator including a Kalman filter. The relative angle estimation logic is responsive to the phase current magnitudes of the switched reluctance machine and develops an angle estimate for each phase. The angle combination logic combines the phase angle estimates to obtain an absolute angle estimate which eliminates ambiguities that would otherwise be present. The estimator utilizes a model of the switched reluctance machine system as well as the absolute angle estimate to form a better estimate of the rotor position and velocity and, if necessary or desirable for other purposes, the rotor acceleration. An instantaneous position generation circuit converts the coarse sampled output of the Kalman filter into a signal having position update intervals which are sufficiently fine to properly control commutation.
While the foregoing approach is effective to provide adequate control for a switched reluctance machine, it is desirable to simplify the control to further enhance reliability and reduce cost.
Accordingly, it is an object of the present invention to provide a commutation apparatus and method for a sensorless switched reluctance machine system which is simple, reliable and low in cost. It is further an object of the present invention to provide such a commutation apparatus and method which relies upon the use of current sensors to achieve commutation without the need for calculating or estimating rotor position.
A further object is to provide a commutation apparatus and method which controls switches in an inverter coupled to phase windings of a switched reluctance machine based upon the magnitudes of currents flowing in the phase windings of the machine.
These and other objects and advantages are attained by providing current sensors to sense the phase current magnitudes flowing through the switched reluctance machine and operating a finite state machine to open or close each switch in each inverter leg based upon the sensed current magnitudes. Specifically, the finite state machine controls a pair of switches in each inverter leg such that each machine phase winding is excited or such that the currents flowing through the phase windings freewheel or, alternatively, fly back during operation in a generating mode based upon the magnitude of current flowing through the phase winding. Because there is no need to estimate or calculate rotor position in order to properly commutate currents flowing through the phase windings, the commutation control circuitry can be greatly simplified, leading to high reliability and lower costs.
These and other objects, advantages and novel features of the present invention will become apparent to those skilled in the art from the drawings and following detailed description.